Ever since the early days of Pokemon there have been both TM's and HM's. They are both move's that you teach to your Pokemon, in the olden days TM's once used would go away, now they stay and you can use them as long as you like. HM's have always been able to stay, but you can not remove a HM from your Pokemon's move set once its been learnt. You can only remove it from a move tutor who is located somewhere in the game, this kind of made sense in the old games. Because before you needed moves like Cut and Strength to progress through the game, in the latest game I have got both Strength and Cut and have not needed to use it. I presume you will need surf but thats actually a good move, where as Cut and strength are not all that.
It wouldn't be as bad if we could at least unlearn them when we want, why do they still feel the need to call them HM's and not let us unlearn them?